The invention is based on Japanese Priority Application No. 11-067361, filed Mar. 12, 1999, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to a brake booster pressure control apparatus, and more particularly to a brake booster pressure control apparatus that is appropriate to control the vacuum pressure of a brake booster, which assists a braking operation on a brake pedal by utilizing the vacuum pressure of intake air supplied from an intake pipe of an internal combustion engine to the brake booster.
2. Description of the Related Art
As disclosed in Japanese Laid-Open Patent Application No. 10-167047, a brake booster pressure control apparatus is known. The brake booster pressure control apparatus controls the vacuum pressure of a brake booster that acts on a brake master cylinder, in addition to a pedal depressing force exerted by the vehicle operator on a brake pedal and transmitted to the brake master cylinder. The brake booster produces a braking force that assists the braking operation by the vehicle operator to depress the brake pedal in the vehicle. The pressure of the brake booster can increase the depressing force of the vehicle operator on the brake pedal, which is transmitted to the brake master cylinder. In order to produce an adequate vacuum pressure, the brake booster of a certain type utilizes intake air pressure supplied from an intake pipe of an internal combustion engine to the brake booster.
The brake booster pressure control apparatus of the above-mentioned publication is applicable to a direct injection engine. In the direct injection engine, a fuel injection valve is provided in the combustion chamber of the engine, and the fuel injection valve injects fuel directly into the combustion chamber. For example, during a low-load operation of the direct injection engine, the throttle valve in the intake pipe is set in a valve-open position even when the vehicle operator does not depress the accelerator pedal, and a large amount of intake air can be supplied to the combustion chamber. This allows the direct injection engine to achieve a stratified charge combustion (SCC), which makes it possible to improve the fuel economy during the low-load operation of the engine.
In the case of the direct injection engine, the pressure of intake air in the intake pipe may be lowered when the throttle valve is set in the valve-open position regardless of whether or not the vehicle operator depresses the accelerator pedal. With the lowered level of the intake air pressure, it is difficult to obtain an adequate level of the vacuum pressure through the input pipe to the brake booster. The pressure of the brake booster may become insufficient to assist the braking operation. To eliminate this problem, the brake booster pressure control apparatus of the above publication starts execution of a pressure control routine in which the valve-open position of the throttle valve is adjusted in a direction to raise the intake air pressure, if a detected pressure of the brake booster exceeds a pressure-control starting pressure.
However, in the case of the above-mentioned brake booster pressure control apparatus, if the valve-open position of the throttle valve is changed to a smaller valve-open position, the driving force output by the engine is lowered, and this causes a fluctuation of the engine torque. Moreover, in such a condition, the combustion state of the engine is transferred from the SCC mode to a stoichiometric combustion (SMC) mode, and this causes the fuel economy to be worsened. In order to eliminate such problems, the above-mentioned brake booster pressure control apparatus varies the pressure-control starting pressure in response to a detected vehicle speed.
As described above, the execution of the pressure control routine is started by the above-mentioned brake booster pressure control apparatus if the detected brake booster pressure exceeds the pressure-control starting pressure. As the pressure-control starting pressure is varied in response to the detected vehicle speed, the execution of the pressure control routine can be started at a time that is suitable to prevent the fluctuation of the engine torque or the worsening of the fuel economy.
Generally, when an ABS (antilock brake system) control is executed, repetitive fluctuations of the pressure of brake fluid in the brake system take place, and such hydraulic pressure fluctuations cause movements of the brake pedal. Hereinafter, such movements of the brake pedal will be called the brake pedal vibrations. The pressure of the brake booster is mostly consumed when the stroke of the brake pedal changes considerably. When the brake pedal vibrations take place, the energy consumption of the brake booster will be increased. In other words, the energy consumption of the brake booster when the ABS control is executed is larger than that of the brake booster when a normal braking operation is executed.
However, the above-mentioned brake booster pressure control apparatus does not take account of the increased energy consumption, such as with the execution of the ABS control, although it executes the pressure control routine or varies the pressure-control starting pressure as described above. It is difficult for the above brake booster pressure control apparatus to maintain adequate brake booster pressure for the energy consumption of the brake booster, which is increased during the execution of the ABS control or upon occurrence of the brake pedal vibrations.
In order to overcome the problems described above, preferred embodiments of the present invention provide an improved brake booster pressure control apparatus that effectively maintains adequate brake booster pressure for the energy consumption that is increased when the brake pedal vibrations take place.
According to one preferred embodiment of the present invention, a brake booster pressure control apparatus, which is provided for controlling the pressure of a brake booster, the brake booster assisting a braking operation on a brake pedal by using an intake air pressure, the brake booster pressure control apparatus including: a brake-pedal vibration detecting unit which outputs a signal indicating the occurrence of vibrations of the brake pedal; and a pressure control unit which controls the pressure of the brake booster based on the signal output by the brake-pedal vibration detecting unit.
In the brake booster pressure control apparatus of the preferred embodiment, the brake-pedal vibration detecting unit outputs the signal indicating the occurrence of the brake pedal vibrations, and the pressure control unit controls the brake booster pressure based on the signal output by the brake-pedal vibration detecting unit. The brake booster pressure control apparatus of the present invention is effective in providing adequate brake booster pressure for the energy consumption that is increased when the brake pedal vibrations take place.